This invention relates to measurement systems for determining the boundaries between phases, in particular to such systems in which the boundaries between at least three phases have to be located and especially to the location of gas-oil and oil-water boundaries in separation vessels in oil production installations.
In oil production it is often necessary to separate aqueous, oil and gas phases that form the flow from a production well. Water and gas are often naturally co-produced with oil and, as oil fields approach the end of their useful life, water is often injected into the oil bearing strata to maintain the production of oil and this results in the stream from the production wells including an increasing proportion of water. Minerals e.g. sand and heavy oil or tar materials e.g. asphaltine, may also be present in the flow from the well. This gives a product stream which needs to be separated before further processing.
Typically such separation is carried out in a separation system which may include pre-separation means such as a cyclone to separate much of any gaseous phase present from the liquid phases and which usually includes a separation vessel in which the fluid flow is slowed and rendered less turbulent e.g. using baffles, and then allowed to separate into layers which are then separately taken from the separation vessel. The means for removing the respective phases are usually fixed within the separation vessel which typically operates at superambient pressure typically up to several times ambient pressure e.g. from 2 to 10 bar absolute (0.2 to 1 MPa) The fixed positioning of the means for removing the respective phases means that control of the separator to maintain satisfactory operation is by way of controlling the various flow rates (inflow and outflow) so that the levels of the various phases in the separator are maintained suitably to enable their ready removal from the separator. The separation of the phases may be made difficult in practice by foam formed by liquid and gas phases and dispersions or emulsions of oil and aqueous phases. The presence of foam or emulsions makes the inter-phase boundaries less definite and thus makes overall control more difficult. The operation of such separators is complicated because it is difficult to determine the location of the phase boundaries from outside the separator. The nature of the materials and the pressure under which separators operate make it impractical to use direct visual means e.g. sick; glasses, and instrumental optical systems are not satisfactory.
The present invention adopts measurements of the adsorption or dispersion of ionising radiation as a means of measuring the density of the medium at a number, usually many, levels in a multi-phase mixture, as in an oil separator, thereby enabling a density profile to be established, from which the position of the phase boundaries and, if desired, the thickness of any interphase regions e.g. of foam or dispersions or emulsions, can be determined.
Accordingly the present invention provides a density profiler for measuring a density profile of a medium including at least two liquid phases and a gaseous phase which profiler comprises:
1. an axially distributed array of sources capable of providing at least 1 collimated beams of ionising radiation;
2. an axially distributed array of radiation detectors, each detector being associated in use with a respective one of the said beams of ionising radiation and producing an output signal in response to the incidence of the ionising radiation;
3. means for analysing the detector output signals to determine the density of the medium traversed by the beams of radiation in passing from the source array to the detector array.
The invention specifically includes:
an oil separator which incorporates a density profiler of the invention in which in use an input oil containing stream includes oil, water (aqueous phase) and gas an the density profiler is positioned to measure the density of oil, aqueous and gas phases;
a method of measuring the density profile of a medium including oil, aqueous and gas phases in which a density profiler of the invention is positioned in a region of the medium in which the different phases are at least partially separated;
a method of controlling an oil separator including a density profiler of he invention, in which the position of the phase boundaries is determined from a density profile measured according to the invention and the inlet flow rate to and/or one or more outlet flow rates from the separator are controlled to maintain the position of the phase boundaries within predetermined limits;
a method of controlling an oil separator including a density profiler of the invention, in which the thickness of the interphase regions is determined from a density profile measured according to the invention and the concentration of chemicals added to the separator to reduce the formation of interphases is controlled to maintain the thickness of the interphase regions within predetermined limits.
The invention further includes a combined radiation source holder and collimator, suitable for use in the density profiler of the invention, in which a source holder is a rod having plurality of, particularly radial, holes adapted to receive radiation sources and, arranged telescopically, desirably substantially coaxially, with the rod, a tube, made of radiation adsorbent material, which has transmission holes in it, the rod and tube being moveable, particularly axially moveable, relative to one another so that in a first position each source is in registry with at least one transmission hole aligned to provide a oath along which radiation from the source traverses the thickness of the tube to produce a collimated beam of radiation which is projected laterally relative to the rod and tube, and that in a second position each source is masked by a portion of the tube so that no collimated beam of radiation is generated.